The present invention is related to a communication method of a contactless ID card and also to a circuit for realizing this communication method, and is to provide a communication method with employment of a code capable of readily separating data from a clock, and to provide a circuit for realizing this communication method, and further, to provide a transmitter, a receiver utilized in this communication method, and a communication system.
Very recently, a strong need is made to replace bar code systems by IC cards in view of information security, reliability, highly valuable services, and higher automatical operations. These bar code systems are utilized in magnetic cards such as cash cards and commuter passes, and also in logistic systems. These IC cards build an integrated circuits (ICs) therein. Among these cards, some cards are capable of transmitting/receiving data as well as capable of supplying electric power between data readers and these cards in the manner of electromagnetic waves, namely in the wireless manner without direct contact between them. These wireless cards are referred to as “contactless ID cards”.
The contactless ID cards are classified into a close coupling type ID card used, while being coupled to a sensor; a proximity type ID card used, while being separated from a sensor by approximately 20 cm; and a remote type ID card used, while being separated from a sensor by approximately 50 cm. The close coupling type ID cards are mainly applied to credit cards. The proximity type ID cards are applied to commuter passes and ID cards. The remote type ID cards are applied to TAGs of logistic systems. The close coupling type ID cards and the proximity type ID cards receive supplies of information and electric power by way of mainly magnetic fields. The remote type ID cards receive supplies of these information and electric power by way of electromagnetic waves. Among 3 sorts of these contactless ID cards, in particular, the remote type ID cards own the following development problems since the received electric power is very weak. That is, more specifically, the remote type ID cards are operable under low power consumption, and also the electric power is supplied in high efficiencies.
FIG. 2 shows an example of a remote type contactless ID card system. An IC card is arranged by an antenna, a diode for detection, a diode for a power supply voltage generator, a preamplifier, a clock generator, a decoding circuit, a logic control circuit, a memory, a driving FET for answering, and so on. An amplitude modulation signal containing information of a clock and data is transmitted from a reader/writer. When a signal is received, electric charges are stored into a power capacitor, and then the IC card is operated by using a voltage appearing across the capacitor as a power supply voltage. The signal detected by a detector is separated into the data and the clock by the clock generator and the decoding circuit, which are processed by the logic control circuit. When the IC card sends the answer to the reader/writer, an impedance of the antenna is modulated by the driving FET for answering.
In the conventional remote type contactless ID card systems, the Manchester code is applied so as to communicate the data between the ID cards and the data readers/writers, as described in “A Low-Power CMOS Integrated Circuit for Field-Powered Radio Frequency Identification Tags”, by D. Friendman et.al., IEEE ISSCC97, SA.17.5, 1997. FIG. 3 indicates a modulation waveform by the Manchester code. In the Manchester code, a transition from “H (high voltage)” state to “L (low voltage)” state is allocated to 1, and a transition from “L (low voltage)” state to “H (high voltage)” state is allocated to 0. When time of “H” is not equal to time of “L”, namely a duty ratio is not equal to 50%, a DC offset is produced by data. When a level of a received signal is varied, or fluctuated, this fluctuation mainly causes reading errors. In accordance with the Manchester code, the time of “H” is set to be equal to the time of “L” so as to realize the signal having the duty ratio of 50%, and the code suitable for the communication is realized without the occurrence of the DC offset.
However, in order to decode the Manchester code corresponding to the above-described prior art, since “0” and “1” are determined based upon the appearing order of the H/L states, the respective H/L states are required to be detected. In other words, it is required to employ such a clock signal having a time period shorter than, or equal to a half time period of a single code. Also, as indicated in FIG. 3, since an interval between rising-edge transition timing and falling-edge transition timing is varied in accordance with data, the phase-locked loop and the oscillator are required so as to produce the clock signal, and convergence of clock requires lengthy time. The locking condition of the phase-locked loop is given by that the self-running frequency of the oscillator is defined within +50% and −50% of the frequency of the reference signal. To satisfy this condition, the temperature, the power supply voltage, and the process fluctuation of the device must be canceled, and a complex reference circuit is required, and thus, the consumed current is increased. In such a case that the communication is temporarily interrupted due to adverse influences caused by electromagnetic wave conditions, there is such a problem that the convergence of clocks is prolonged, and thus, lengthy locking time is required.